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Possibly needing prop adjustment


Rich

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Finally got up today and think a prop adj. is necessary.

 

Take of at airport elev. 1700 MSL, 37*F. Climb to 2,500 MSL 4930-4940 at 70 KIAS.

 

Straight and level at 5,000 MSL, 50*F (temp inversion in northern PA today) 5300 5310 MAX WOT.

 

I think a 1/2 to 1 degree reduction in pitch is necessary.

 

DUC Helices Swirl, now pitched at 23.5 degrees.

 

 

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  • 4800 RPM - 103 KIAS - 108 KTAS
  • 5000 RPM - 106 KIAS - 112 KTAS
  • 5200 RPM - 112 KIAS - 119 KTAS
  • 5700 RPM (WOT) - 122 KIAS - 130 KTAS

 

These numbers have a lot more meaning if a density altitude is included.

 

Ok, a good reason to plan an other flight this WE to perform some other measurements with DA included...

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Always set you prop pitch for your average altitude. It won't be as good to set a prop for 2K if you always fly at 8K.

 

And isn't that the rub?  Who flys at the same altitude all the time?  Adjust it today and again tomm. and the next day and the day after that?

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That's it. Set it and forget it. 

 

If you usually fly from 4K to 9K then set it for around 6.5K-7K We tend to spend more time way up off the ground than within 500' of it. It's just a balance between all our in flight factors since we can't make in flight adjustments. Sometimes you'll be lower and sometimes higher. 

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Since you didn't read it, he said set it for the AVERAGE altitude. It is more like, "set it and forget it".

 

Since you didn't read what I said let me say in more plainly.  Do you fly at sea level or 4k or 8k or 10k?  Possibly all, correct?.   The factory has already set the prop....  How are you 'improving' what the factory has done again? 

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Since you didn't read what I said let me say in more plainly.  Do you fly at sea level or 4k or 8k or 10k?  Possibly all correct?.   The factory has already the prop....  How are you 'improving' what the factory has done again? 

 

 

The factory pitched my prop so coarse that 98 house power was not available on climb which is ok but 93 hp was not available for curise and enve 75hp was not available at 7,500'.

 

My 100hp engine was effectively detuned to a 90hp model.  By flattening the pitch I now have all available power for cruise flight  :)

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Seems I remember redline to be 5800, not 5700. Close enough to pay attention but 5700 is just fine for up to 5 minutes.

 

From the POH:   5500-5800 for 5 minutes only in the yellow.  I cruise at 5300rpm for that reason. 

 

Also, what is 'average' altitude?  Each day density altitude changes...any given MSL is going to vary often by hundreds of feet.  Are you planning to change your prop pitch for each season of the year too?

 

It's a statistical  nightmare and no one is going to 'improve' what the factory sets.

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You really don't understand the concept of "average altitude"? If you cruise sometimes at 7500, others at 8500, and others at 9500, your average will be 8500ft. You will get best efficiency tuning the prop for 5500-5700rpm WOT at that altitude. You will not be as efficient at other altitudes, but since you are spending most of your time in that range, it will be close enough.

 

What if you make a flight at 3500ft? Well then, you will be less efficient there. But so what? You are rarely flying there...and EVERY prop setting is a compromise. Do you think that the factory setting is magical and somehow optimally efficient at more than one altitude? Spoiler alert: it's not.

 

I gained eight knots in cruise AND 300fpm climb over the factory setting following Roger's advice on this. If you ignore the collected experience of other owners and mechanics that set these airplanes up for a living, you deserve whatever results you get.

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You really don't understand the concept of "average altitude"? If you cruise sometimes at 7500, others at 8500, and others at 9500, your average will be 8500ft. You will get best efficiency tuning the prop for 5500-5700rpm WOT at that altitude. You will not be as efficient at other altitudes, but since you are spending most of your time in that range, it will be close enough.

 

What if you make a flight at 3500ft? Well then, you will be less efficient there. But so what? You are rarely flying there...and EVERY prop setting is a compromise. Do you think that the factory setting is magical and somehow optimally efficient at more than one altitude? Spoiler alert: it's not.

 

I gained eight knots in cruise AND 300fpm climb over the factory setting following Roger's advice on this. If you ignore the collected experience of other owners and mechanics that set these airplanes up for a living, you deserve whatever results you get.

 

You don't understand the concept of varying Density Altitude?  Or taking off on a 6k msl home field to 10k cruise and then to a sea level airport in the same day? 

 

If you are so desperate to play with your propeller pitch then get a private pilots license and get yourself into a Cirrus SR22t.    Instead of an independent lever control, it uses a cam actuator in the throttle to increase the prop pitch as you push the throttle toward the stop, increasing the prop RPM to its limit of 2,700.

 
Heck, if you are handy, maybe you can rig that up on your little SW?  No one will know...
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You don't understand the concept of varying Density Altitude?  Or taking off on a 6k msl home field to 10k cruise and then to a sea level airport in the same day? 

 

If you are so desperate to play with your propeller pitch then get a private pilots license and get yourself into a Cirrus SR22t.    Instead of an independent lever control, it uses a cam actuator in the throttle to increase the prop pitch as you push the throttle toward the stop, increasing the prop RPM to its limit of 2,700.

 
Heck, if you are handy, maybe you can rig that up on your little SW?  No one will know...

 

 

I don't need a Cirrus, what does that have to do with anything?!?  There is a reason FD put an adjustable prop on the CT, and it's not so that we could buy a Cirrus if the factory prop setting isn't correct for how we fly.  

 

Varying density altitude does not change *anything* I have said.  You are not trying to get an optimal setting at every possible altitude (density or MSL), because doing that is impossible.  The goal is to get the best compromise for the normal way you use YOUR airplane.  The factory setting is a wild-ass guess for how an average owner might use his plane, and at some arbitrary guessed altitude that you don't cruise at.  

 

Based on your posts, you spend a lot of time flying above 8000 feet.  Do you think that FD sets up their prop to be anywhere near efficiency at that altitude, when most CT owner/pilots fly at 5000ft or less?  So you set your prop up for 8500ft and one day the density altitude is 10,000ft at 8500ft...do you think you are closer to correct with a prop setup for 8500ft or for 5000ft?  This is not rocket science.

 

If the DA is consistently higher than your cruise altitude where you fly, guess what?  You can set your prop up for that too.  There is ONE single point where your prop is optimally efficient, moving that point as close to how you operate the aircraft as possible is the most practical way to use your airplane.  If you don't do that, you are leaving performance and fuel efficiency on the table.

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Over the years the CT has proven to be able to fly with many different prop settings. you want to set the prop pitch to get the best power, efficiency, or best economy in the conditions that you fly the most. If set this way you will be OK when you go to the other conditions, just not the best. Early on in the life of the CT in the USA the props were pitched where the engine would only turn 5200RPM at full throttle level flight. This put to much load on the engine and did not offer the best performance, and is also why re-pitching the propeller became so important. The other reason for checking and adjusting the prop pitch is to get smoother engine operation. If the blade angles are not correctly matched the engine will feel rough.

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Over the years the CT has proven to be able to fly with many different prop settings. you want to set the prop pitch to get the best power, efficiency, or best economy in the conditions that you fly the most. If set this way you will be OK when you go to the other conditions, just not the best. Early on in the life of the CT in the USA the props were pitched where the engine would only turn 5200RPM at full throttle level flight. This put to much load on the engine and did not offer the best performance, and is also why re-pitching the propeller became so important. The other reason for checking and adjusting the prop pitch is to get smoother engine operation. If the blade angles are not correctly matched the engine will feel rough.

 

That matches my experience.  5200rpm was WOT on my 2007 CTSW when I bought it (used).  It could not break ~110kt and climb was a bit weak.  I had it repitched per Roger's recommendations and now get ~120kt at 5400rpm.  I also gained about 300fpm in climb. 

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Once I realized that my CT climbs well and climbs even better after I optimize my prop pitch for cruise I then logically decided to optimize for cruise not climb.

 

Considering that best speed is available at 7,500' provides a lot of motivation to pitch for 5,500 @ 7,500' @ WOT so best speed can be achieved.

 

Adding a personal 75% power cruise limitation locks in 5,500 @ 7,500' @ WOT as a floor or the coarsest pitch that makes sense.  You could go coarser for higher speeds at lower altitudes but those power settings are not comfortable and they exceed 75% power.  IMHO this is the most reasonable pitch for the majority of CTs.

 

For me pattern altitude at home is higher than 7,500 so 10,000'+ seems a bit better.  I'll leave my pitch where it is unless I move to Florida and even then the change would be small.

 

"Normal" cruise altitude might vary a lot depending on where we live and fly yet our pitches once optimized are probably pretty close with the difference being that the low altitude fliers probably don't cruise WOT. 

 

Edit:  As well, high altitude fliers that rely on econo mode (lean power settings below 92% throttle) don't cruise WOT either.  At some altitude, 13,500? even the cheapskates will firewall it.

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With increasing altitude drag is reduced  but so is available power.  At 7,500' DA you still have 75% power available which is typically maximum cruise power but you have a meaningful reduction in drag.

 

Lower is slower due to additional drag and higher is slower due to reduced available power.

 

7,500' is an approximation, 6,500 and 8,500 give similar results.  When comparing available performance at different DA I assume prop is pitched to realize 5,500 @ WOT at that altitude.

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6-8k is an altitude that is generally best performance for all non-turboed otto cycle engines (reciprocating engines) which exhibit constant volume intake properties. Atmospheric density is non-linear in regards to altitude, so what occurs is a substantial loss of pressure at first, and then slows as you go higher and higher. At low altitudes, we tend to use less throttle as to restrict the amount of incoming air into the engine (to not drive the engine too hard), but at higher altitudes where the air is thinner, we open the throttle valve more to draw in a higher volume of air to maintain power since the air density is lower (fuel consumption in a perfect system would remain the same). Eventually there is a point where the loss of atmospheric density cannot be made up by simply opening the throttle more, and thus you've hit the aircraft's "sweet spot" regarding your desired cruise speed. In addition, the reason why we want to climb reasonably high, is that we want as thin of air as possible on the airframe too, since it results in a higher airspeed due to decreasing friction as a result of fewer air molecules acting on the aircraft, thus it speeds up to reestablish equilibrium. Putting all of this together, this sweet spot is lower and lower as you want to use more and more power, and gets higher and higher if you are at a more gentle pace (it's not too significant, just a note). Once again, this is for non-turboed constant volume engines, like the otto cycle (recip) engines.

 

There are probably other factors that I am missing, but that's the gist of it.

 

Turboed engines typically just climb as high as their critical altitude (when the turbo is at max efficiency), and maybe then some depending on desired cruise speed. Brayton cycle engines (jet engines) have a bunch of other factors to determine their altitude (these are called constant pressure engines), but essentially are like turboed engines, since they use a compressor section that acts just like a turbo, and bleed air valves that act like waste gates.

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